L.S.,
I guess this should be simple but I seem only to be successful in part.

From I website I get data which in stead of 
<tag> returns &lt;tag&gt;

I wish to replace those &lt; en &gt;  
However a standard substitute does not work. Escaping the & with \& does not work either.

 

Does someone have a solution ?

a standard substitute does not work.

Not sure what you mean by that. A "standard substitute" like this:

Substitute ( text ; [ "&lt;" ;"<" ] ; [ "&gt;" ; ">" ] )

will return "<tag>" when text contains "&lt;tag&gt;". There are no reserved characters here and nothing to escape.

 

You will probably want to add:

[ "&" ; "&" ]

to that.

 

--
P.S. There's probably a better way to get from the origin to the destination - but you're not showing us the big picture.

Edited April 21, 201510 yr by comment

Doesn't

Substitute ( "&lt;span&gt;" ; [ "&lt;" ; "<" ] ; [ "&gt;" ; ">" ] )

return "<span>"?